<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app">
        <input type="text" @input="first" v-model="firstname">
        <input type="text" @input="last" v-model="lastname">
        <input type="text" v-model="allname" >
    </div>



    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.13/dist/vue.js"></script>
    <script>
        var vm = new Vue({
            el:"#app",
            data:{
                firstname:'',
                lastname:'',
                allname:''
            },
            methods:{
                first(){
                    console.log(this.firstname)
                    this.allname=this.firstname+"."+this.lastname
                    console.log(this.allname)
                },
                last(){
                    this.allname=this.firstname+"."+this.lastname
                }
            }

        })
    </script>
</body>
</html>